1. Field of the Invention
The present invention is directed to a method for undertaking a format conversion of image sequences from a first pixel raster into a second pixel raster.
2. Description of the Prior Art
A multiplicity of picture (image) formats (pixel rasters), which are mostly incompatible with one another, exist for the recording, transmission and reproduction of image sequences. Depending on the application, the recording takes place with varying height/width ratio and with varying local and lateral resolution. In addition, scanning takes place, for example, either using a line interlace method or progressively, line-by-line. Corresponding differences exist during reproduction of the picture.
A central problem is format incompatibility in the linking of the "television world" with the "computer world." With the appearance of multimedia applications, however, it becomes necessary to eliminate this incompatibility and to carry out a picture format conversion from "interlaced TV " at 50 hertz or 60 hertz field frequency into the computer format (progressive mode with image frequencies between 60 and 100 hertz).
This problem has not yet been taken into account, however, in the computer cards currently available commercially. On the contrary, the two fields of a frame are always written into an image memory and the latter is read out sequentially. In stationary areas of the image, this method poses no problems, but in the case of movement within the image, disturbing effects (i.e. comb-like distortion of vertical edges or double contours during horizontal movement, scrambling of a moving object and background during vertical movement, scrambling of finely structured image regions during all movements) occur because of the time displacement of the two fields. This method is not acceptable for high-quality reproduction of video on computer terminals. A further, very simple solution is to supplement the missing lines in each field by vertical interpolation. Although the effects mentioned above are thereby avoided, a loss in the vertical resolution now occurs, i.e., the images are not sharp in the vertical direction in all areas. Even the combination of the two methods by moving-object segmentation still fails to provide a satisfactory result since the impression of lack of sharpness in moving objects is retained.
Solutions to this de-interlacing problem have been suggested in the literature. Relatively simple methods use linear or nonlinear (median) filtering for a positional time interpolation (i.e., P. Haavisto, J. Juhola, and Y. Neuvo, "Motion Adaptive Scan Rate Up-Conversion," Multidimensional System and Signal Processing, Vol. 3, No. 2, pp. 113-130, May 1992; M. Weston and D. M Ackroyd, Fixed Adaptive and Motion Compensated Interpolation of Interlaced TV Pictures in International Broadcasting Convention 1988, IEE Conference Publication No. 293, pp. 220-223, Sept. 1988). A further method is based on a direction-dependent interpolation using edge information (i.e. T. Doyle and M. Looymans, "Progressive Scan Conversion Using Edge Information," Proc. 1989 HDTV-Workshop, Turin). The best results are achieved with movement-compensating methods (i.e., M. Ernst; Motion Compensated Video Processing for Studio Applications) Conf. Les Assists des Jeunes Chercheurs, Tokyo, 8-12 June 1992).
In the last-mentioned methods, any movement of objects occurring in the scene is estimated, and an interpolation is carried out taking into consideration the movement vectors determined in this way. These known methods are, however, extremely computationally intensive and are consequently also expensive to implement in terms of hardware. Specifically, compared with the movement estimation for the moving picture coding, the vectors have to be determined not only block by block but also individually for each pixel. Furthermore, reliability tests have to be carried out so that the vectors also describe the actual movement as accurately as possible.